In our studies to date we have identified insulin and lactogen receptors in rat liver Golgi elements and have shown that they closely resemble those in the plasma membrane. Data have been obtained suggesting that the Golgi receptors are precursors for those of the plasma membrane. The demonstration that intact hormone concentrates at the intracellular sites suggests that these sites may have a functional relationship to hormone action. Using in vivo radioautography we have identified insulin receptors in a number of target tissues including pancreas, gut, and adrenal. In the coming year we will evaluate: (1) The time course of uptake of 125I-insulin into a variety of apparent target tissues, (viz, pancreas). (2) The uptake of labeled hormones into subcellular compartments other than Golgi (viz. lysosomes). (3) The regulation of the rat hepatic lactogen receptor in hyperprolactinemic and hypoprolactinemic states in both plasmalemma and Golgi subcellular fractions. (4) The regulation of the hepatic insulin receptor in hyperinsulinemic states. (5) The characteristics and regulation of receptors for growth hormone and somatomedin in Golgi elements.